Config: Flag config uidgid entries

Uidgid entries parsed from configuration files now has prefix
(uidgid.config.) so they are distinguishable from dynamically added
entries. Entries added from config file are pruned on reload if no
longer exists in config file (dynamic one stays unaffected). Also whole
uidgid.config. prefix is made read only.

This make PCMK work again after configuration reload is called.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>